1. Field of the Invention
The present invention relates to a light source and a liquid crystal display (LCD) device using, particularly to a light source device using a light emitting diode (LED) and an LCD device using the same.
2. Description of the Related Art
An LCD device which is used for a personal computer or a mobile phone includes an LCD panel and a sheet-like light source device. The LCD panel has a structure in which a liquid crystal layer is interposed between two substrates, and-the sheet-like light source device is provided on the rear side of the LCD panel.
As a light source of a sheet-like light source device, a linear cold-cathode tube has been conventionally used. However, because of the long lifespan and excellent light emission property of an LED, the LED has been increasingly used as a light source of a sheet-like light source device.
In the case of using an LED as a light source for backlight of a medium- or large-sized display device, a large number of LEDs are needed for the purpose of obtaining necessary brightness. Moreover, a plurality of LEDs, which emit different lights of wavelengths corresponding respectively to red, green and blue colors, are used for the purpose of obtaining white illumination light.
In the case of using LEDs of these three colors, it is necessary that these colors are mixed and emitted to the side of a display surface without color non-uniformity. For this reason, a mixing light guide plate is needed in addition to an ordinary illuminating light guide plate. Examples of an LCD device using such LEDs as light sources are disclosed in Japanese Patent Application Publication No. 2004-199967 and Japanese Patent Application Publication No. 2004-118205 (corresponding to US Patent Application Publication No. 2004/0061810 A1). FIG. 1 is a cross-sectional view showing a configuration of an LCD device of this type.
As shown in FIG. 1, an LCD device includes an LCD panel 108 and a light source device 120 for illuminating the LCD panel 108. The light source device 120 mainly includes LEDs 101 as light sources, a mixing light guide plate (hereinafter referred to as a mixing plate) 104 and an illuminating light guide plate (hereinafter referred to as an illuminating plate) 105. The mixing plate 104 receives light emitted respectively from the LEDs 101 at one end thereof, mixes colors respectively of the light therein to generate white light, and emits the white light from the other end thereof. The illuminating plate 105 receives the light emitted from the mixing plate 104, and then emits the light from the entire plane thereof to illuminate the LCD panel 108. The LEDs 101 and a reflector 103 are provided to the vicinity of an entrance surface of the mixing plate 104. As the LEDs 101, three kinds of LEDs are used, which emit different lights of wavelengths corresponding respectively to colors of red (R), green (G), and blue (B).
The reflector 103 reflects light generated from the LEDs 101, and introduces the light into the mixing plate 104. A reflector 106 is provided to the side of an emitting surface of the mixing plate 104. The reflector 106 reflects light to an entrance end surface of the illuminating plate 105. The reflector 106 is provided in a manner that the reflector 106 surrounds the emitting surface of the mixing plate 104 and one side surface of the illuminating plate 105. Each of the reflectors 103 and 106 has a mirror-finished inner surface to reflect light efficiently. The LCD panel 108 is provided to a display surface of the illuminating plate 105. Optical sheets 107 are provided between the illuminating plate 105 and the LCD panel 108. The optical sheets 107 is configured of a diffusion sheet, a protection sheet, a prism sheet and the like. The optical sheets 107 diffuses or collects light emitted from the illuminating plate 105 so as to illuminate the LCD panel 108.
A reflective sheet 109 is provided at a rear surface of the illuminating plate 105. The reflective sheet 109 is bent at an end portion of an aluminum frame 110, and is fixed to an upper surface of the mixing plate 104 with a fixture 1.11. The reflective sheet 109 reflects light emitted from the rear surface of the illuminating plate 105, thus causing the reflected light to enter into the illuminating plate 105 again. In this manner, the reflective sheet 109 is used to improve the utilization efficiency of light in the illuminating plate 105.
Next, with reference to FIG. 1, the propagation of light in the LCD device will be described. Light from the LEDs 101 enters into the entrance surface of the mixing plate 104 directly or by being reflected by the reflector 103. The light having entered into the entrance surface of the mixing plate 104 propagates in the mixing plate 104 to be mixed. The light which has become white light after being sufficiently mixed is emitted from the emitting surface of the mixing plate 104, and enters into the reflector 106. The reflector 106 reflects the light emitted from the mixing plate 104 to the illuminating plate 105. The light having entered into the illuminating plate 105 uniformly spreads over an entire surface and is emitted from the emitting surface (illuminating surface) which is a surface on the side on which the optical sheets 107 and the LCD panel 108 are provided. The light emitted from the rear surface of the illuminating plate 105 is reflected by the reflective sheet 109, thus entering into the illuminating plate 105 again.
However, in an LCD device having the LEDs 101 as light sources, as shown FIG. 1, a plurality of LEDs of R, G and B are used. And a color non-uniformity is caused in the illuminating surface of the illuminating plate 105 due to variations in the intensities of light emitted respectively from the LEDs.
As a method of reducing the color non-uniformity, a method can be conceivable in which an LED driver circuit is provided to every LED, or in which an interval between LEDs, arrangement of each color, length of a mixing light guide plate and the like are changed. However, in the cases of employing the above methods of reducing color non-uniformity, it is necessary to change wiring on a substrate on which the LED is mounted, or to change a driver circuit. As a result, there is a problem that the structure of backlight becomes complicated and therefore the costs of manufacturing the backlight increase.